The present invention relates to a method for positioning a photosensitized substrate for printing a pattern on the mask onto the photosensitized substrate in an exposure apparatus used in a photolithographic process for fabricating semiconductor devices, image pick-up devices (such as charge-coupled devices), liquid crystal displays, thin film magnetic heads or the like. In particular, it relates to such a positioning method suitable for performing a coarse alignment (or prealignment) operation of a photosensitized substrate on a stage in an exposure apparatus.
In any of various exposure apparatuses used for fabrication of semiconductor devices, liquid crystal displays or the like including a projection exposure apparatus (such as a stepper) and a proximity printing exposure apparatus, in order to transfer a circuit pattern formed on a mask or reticle onto a photoresist film formed on a photosensitized substrate such as a wafer (or a glass plate, etc.) with high registration, it is required to establish alignment between the reticle and the wafer with precision.
For this purpose, there have been proposed various types of alignment sensor systems including: xe2x80x9claser step alignment (LSA) typexe2x80x9d in which a wafer has an alignment mark formed thereon which comprises a linear array of dots, and a laser beam illuminates the alignment mark to produce diffracted or scattered beams, which are used to detect the position of the alignment mark (such as disclosed in U.S. Pat. No. 5,243,195); xe2x80x9cfield image alignment (FIA) typexe2x80x9d in which an image-sensing device is used to take an image of an alignment mark which is illuminated by illumination light having a continuous spectrum of a wide wavelength range obtainable from a halogen lamp, and the picture data of the image is subjected to an image processing to measure the position of the alignment mark; and xe2x80x9claser interferometric alignment (LIA) typexe2x80x9d in which a wafer has an alignment mark formed thereon which comprises a diffraction grating, two laser beams having different frequencies with a small difference between them illuminate the alignment mark from different directions to produce two diffracted beams interfering with each other, and the position of the alignment mark is determined from the phase of the interference. There have been also proposed various alignment techniques which may be categorized into three types including: xe2x80x9cthrough-the-lens (TTL) typexe2x80x9d in which the position of the wafer is measured through a projection optical system; xe2x80x9cthrough-the-reticle (TTR) typexe2x80x9d in which the relative position of a wafer with respect to a reticle is measured through both a projection optical system and the reticle; and xe2x80x9coff-axis typexe2x80x9d in which the position of the wafer is measured directly, or not through a projection optical system.
By using any of these alignment sensor systems to detect respective positions of two points on a wafer which is placed on a wafer stage, the rotational position (or rotational angle) of the wafer may be determined in addition to the position of the wafer with respect to the translational displacement. Alignment sensor systems usable for determining the rotational angle of a wafer include LIA-type system using TTL-type technique, LSA-type system using TTL-type technique, FIA-type system using off-axis-type technique, and others.
The exposure apparatus is required not only to have the ability of establishing alignment with precision between a reticle and a wafer by using the detection results obtained from the alignment sensor, but also to quickly establish such alignment so as to keep high throughput (the number of wafers that can be processed per unit of time). Thus, it is needed to realize highly effective operations in all the steps performed in relation to the exposure apparatus, from a transfer operation of a wafer onto the wafer stage to an exposure operation. Here, we will describe with reference to FIG. 1 the operations in the wafer loading step preceding the final wafer alignment step, as performed in a typical prior art exposure apparatus.
FIG. 1 shows a part of a wafer stage and the associated elements for a typical prior art exposure apparatus, for illustrating a wafer transfer mechanism. In FIG. 1, a lift device 19 is mounted on an X-stage 11 through a linear actuator 20. A substrate or wafer 6 has been loaded onto the lift device 19 from a wafer transfer unit (not shown). The lift device 19 has three support pins (of which only two support pins 19a and 19b are shown in FIG. 1) extending vertically through openings formed in a material support 9, a xcex8-rotation correction mechanism 8 and a wafer holder 7. The linear actuator 20 acts on the lift device 19 to lower/raise the three support pins, so as to lift down/up the wafer 6 for placing it onto and removing it from the wafer holder 7. Each support pin has at its tip end a hole selectively communicable with a vacuum source, which sucks the bottom surface of the wafer 6 to hold it, so as to prevent the wafer 6 from displacing horizontally upon vertical movements of the lift device 19.
Conventionally, a contact-prealignment process is used in which the peripheral edge of the wafer 6 is pressed against and engaged with a plurality of pins after the lift device 19 is lowered to place the wafer 6 onto the wafer holder 7. This prealignment process establish coarse alignment of the wafer 6 with respect to the rotational and translational offsets before the wafer 6 is held by vacuum suction to the wafer holder 7.
After the wafer 6 is held by vacuum suction and fixed to the wafer holder 7 in this manner, an alignment sensor system of a suitable type, such as the LSA type or the FIA type, is used to detect the alignment marks (search marks) formed on the wafer 6 at positions diametrically opposite to each other and produce detection signals. A movable mirror 13 mounted on the material support 9 and the associated laser interferometer disposed outside of the material support 9 together serve to measure the coordinates of the position of the material support 9. By determining such coordinates when the detection signals is at a peak, the translational errors and the rotational error of the wafer in terms of the wafer stage coordinate system are determined. The xcex8-rotation correction mechanism (or xcex8-table) is driven to vanish the rotational error of the wafer 6, so that the alignment in the rotational direction between the reticle and the wafer 6 (search alignment) is performed.
In this prior art technique, the xcex8-rotation correction mechanism 8 for rotating the wafer is disposed between material support 9 and the wafer 6, while the wafer stage coordinate system is defined with reference to the material support 9. Thus, there have been many problems including unwanted horizontal displacements of the wafer 6 which may occur due to insufficient suction for the vacuum-holding of the wafer by the wafer holder 7, a poor rigidity of the wafer stage due to the provision of various complicated mechanisms on the material support 9, as well as a poor controllability of the wafer stage due to the heavy weight thereof imposed by such complicated mechanisms. These problems could not be solved by disposing a xcex8-rotation correction mechanism under the material support 9 because the incident angle of the laser beam from the laser interferometer into the movable mirror 13 fixedly mounted on the material support 9 would change when the rotation angle of the wafer 6 is adjusted by driving the xcex8-rotation correction mechanism, so that the range of rotation angle of the xcex8-rotation correction mechanism would be limited, resulting in a disadvantage that errors in prealignment could not be corrected unless they are sufficiently small.
Furthermore, in this prior art exposure apparatus, the translational errors and the rotational error of the wafer 6 is detected by measuring the positions of two alignment marks (search marks) formed on the wafer 6 and distant from each other by means of a single alignment sensor system of the LSA or the FIA type, after the wafer 6 is held on the wafer holder 7. However, in order to detect two distant alignment marks by means of a single alignment sensor system, the wafer 6 has to be moved so as to position the alignment marks sequentially into the detection area of the alignment sensor system, and this operation has to be repeated for each of the wafers in one lot, resulting in low throughput of the exposure process. This problem could not be conveniently solved by providing two alignment sensor systems for simultaneous detection of the two alignment marks, because the arrangement of the two alignment sensor systems on the exposure apparatus imposes a limitation on the arrangement of the two alignment marks on the wafer 6, so that it would be difficult for such exposure apparatus to accommodate wafers of different sizes, for example.
In this relation, the last problem could not be conveniently solved by providing an adjustor mechanism for adjusting the distance between the two alignment sensor systems because such adjustor mechanism has to be inherently complicated, is difficult to dispose in the space around the wafer stage where various sensors and other components are closely disposed, and tends to increase the manufacture costs.
Also, various contact-prealignment mechanisms have been used to establish coarse alignment after the wafer has been placed on the wafer holder 7. However, there is an disadvantage that high throughput can not be obtained when the prealignment operation has to be performed after the wafer has been placed on the wafer holder. Nevertheless, it is desirable that any exposure apparatus having a newly proposed prealignment mechanism which may provide high throughput, may be capable of providing the matching with another, existing exposure apparatus having a conventional contact-prealignment mechanism.
The orientation of a wafer can be corrected to meet a predefined reference by performing the prealignment process, in which a noncontact-type prealignment mechanism is used to measure the outer contour of the peripheral edge of the wafer and the orientation of the wafer is so corrected as to coincide with the reference. However, this prealignment process still suffers from a problem: in the case that the alignment marks on the surface of a wafer, which have been formed through a previous lithographic process, offset (in a rotational direction) away from the desired positions that are predefined relative to the geometrical features (such as an orientation flat and a notch) defined by the configuration of the peripheral edge of the wafer, the fine alignment process following the prealignment process may not be performed efficiently, or it may even be impossible at all to perform the fine alignment process so the wafer has to be rejected as a failure.
It is an object of the present invention to provide a method for positioning a substrate which contributes to enhance the rigidity of a wafer stage and reduce the weight of the wafer stage, resulting in that the positioning operation of a wafer upon loading of a wafer by, for example, a wafer loader system onto the wafer stage, can be quickly performed with precision.
It is another object of the present invention to provide a method for positioning a substrate by which when the positioning operation of a wafer is performed using a wafer stage and with reference to the positions of alignment marks formed on a wafer, the positioning operation can be quickly performed without any limitation imposed on the arrangement of the alignment marks.
It is a further object of the present invention to provide a method for positioning a substrate which may achieve high matching accuracy for the prealignment with other exposure apparatus in which a contact-prealignment process is utilized.
It is further object of the present invention to provide a method for positioning a substrate, such as a wafer, in which a fine alignment process may be performed with high efficiency even for such a wafer having its alignment marks the position of which are displaced or offset from the desired positions represented by the geometrical features of the wafer, so as to improve productivity of the products to be fabricated on the wafer, such as microdevices or others.
According to a first aspect of the present invention, there is provided a method for positioning a substrate on a two-dimensionally movable substrate stage, the substrate having a peripheral edge with a cutout formed therein, the method comprising the steps of: (a) transferring the substrate to a loading position above the substrate stage; (b) measuring, at the loading position, positions of a measurement point on the cutout formed in the peripheral edge of the substrate and of another measurement point on the peripheral edge of the substrate, by using a noncontact-measurement technique; and (c) determining a rotational error of the substrate based on the measurement results obtained through the step (b).
In this alignment method, if the cutout formed in the peripheral edge of the substrate comprises a notch as shown in FIG. 6(b), the measurement of the position at the measurement point on the cutout may be preferably made by a two-dimensional image processing unit. Further, for the substrate having a notch, the two-dimensional, positional offsets (translational errors) and the rotational error of the substrate may be detected by performing one-dimensional position measurement at one additional measurement point on the peripheral edge of the substrate other than the measurement point on the notch.
On the other hand, if the cutout formed in the peripheral edge of the substrate comprises an orientation flat, the position measurement may be made at any of the measurement points on the peripheral edge of the substrate by image processing units, and one-dimensional position measurement may be sufficient for the purpose. However, when one-dimensional position measurement is used, the measurement are performed at measurement points including one on the orientation flat and at least two other measurement points (hence at least three measurement points in total) in order to detect the two-dimensional offsets and the rotational error of the substrate. In either case, the two-dimensional offsets may be corrected by adding the offsets to the target position of the alignment in the subsequent search alignment process. By virtue of this method, a rotational correction mechanism on the substrate stage may be eliminated and the accuracy is improved.
According to a second aspect of the present invention, there is provided a method for positioning a substrate on a two-dimensionally movable substrate stage, comprising the steps of: (a) forming on each substrate first and second search marks each for indicating a two-dimensional position; (b) detecting two-dimensional positions of the first and second search marks on a first substrate; (c) determining a rotational error of the first substrate based on the two-dimensional positions detected through the step (b); (d) detecting and storing an at least one-dimensional position of a pattern spaced a predetermined distance from the first search mark on the first substrate, while detecting a two-dimensional position of the first search mark; (e) detecting an offset, from the position stored through the step (d), of the pattern spaced the predetermined distance from the first search mark, while detecting a two-dimensional position of the first search mark on a second substrate on the substrate stage, so as to determine from the offset positional error an offset positional error of the second substrate.
In this positioning method, for the second substrate, the first search mark is positioned in the detection area of a predetermined first alignment sensor system, and then the position of a pattern (such as a street-line) in the detection area of a second alignment sensor system spaced a predetermined distance form the first alignment sensor is compared with the stored position for the first substrate, and any alignment error may be determined form the results of this comparison. Thus, for the second substrate, it is unnecessary to perform the detection of the position of the second search mark, and the detection of the position of the pattern in the detection area under the second alignment sensor system together with the detection of the first search mark by the first alignment sensor system at the same time may be sufficient for establishing alignment of the second substrate, resulting in a reduce time required for the measurement.
According to a third aspect of the present invention, there is provided a method for positioning a substrate on a two-dimensionally movable substrate stage, the substrate having a peripheral edge with a cutout formed therein, the method comprising the steps of: (a) transferring the substrate to a loading position above the substrate stage; (b) measuring, at the loading position, positions of a measurement point on the cutout formed in the peripheral edge of the substrate and of another measurement point on the peripheral edge of the substrate, by using a two-dimensional image processing system and a noncontact-measurement technique; (c) determining, in the observation fields of the two-dimensional image processing system, imaginary points corresponding to reference points which would be used for positioning the substrate on the substrate stage by using a contact-positioning technique; and (d) using offsets, from the imaginary points, of the positions of the measurement points measured by the two-dimensional image processing systems, to make a prediction of a position of the substrate which will be found when the substrate has been placed on the substrate stage.
In this positioning method, the rotational error of the substrate is detected at the loading position of the substrate distant from the substrate stage, so that the rotational error may be corrected through the substrate lift means while the substrate is lowered from the loading position onto the substrate stage. Therefore, it is unnecessary to provide a rotation correction mechanism on the substrate stage side, and thereby the substrate stage may have a relatively simple construction, an improved rigidity and a reduced weight, resulting in that the alignment operation of the substrate may be quickly performed with precision upon loading of the substrate from a substrate transfer system (such as a wafer loader system) onto the substrate stage.
Further, the imaginary points corresponding to reference points which would be used for positioning the substrate on the substrate stage by using a contact-positioning technique, are determined in the observation fields of the two-dimensional image processing system. Also, the offsets, from the imaginary points, of the positions of the measurement points on the photosensitized substrate measured by the two-dimensional image processing system are used to establish alignment of the substrate. Therefore, a high matching accuracy for the coarse positioning (prealignment) process, with another exposure apparatus in which a contact-positioning (prealignment) process is performed, may be obtained.
Further, in this positioning method, if the cutout formed in the peripheral edge of the substrate comprises a V-shaped notch, the measurement points for the two-dimensional image processing system may preferably include one on the cutout and two on respective portions of the peripheral edge of the substrate other than the cutout. In this case, the cutout is a recess such as a notch, and thus the detection of the positions at the three measurement points enables identification of the rotational angle and the two-dimensional position of the substrate.
On the other hand, if the cutout formed in the peripheral edge of the substrate comprises a flat edge portion, the measurement points for the two-dimensional image processing system may preferably include two on the cutout and one on a portion of the peripheral edge of the substrate other than the cutout. In this case, the cutout is a flat cutout such as an orientation flat, and thus the detection of the positions at the three measurement points enables identification of the rotational angle and the two-dimensional position of the substrate.
Furthermore, it is preferable, in order to make the prediction of the position of the substrate which will be found when the substrate has been placed on the substrate stage, to obtain a rotational error and offsets between a position of the substrate which will be found when the substrate has been placed on the substrate stage through the substrate lift means without any rotation effected thereby and a position of the substrate which would be found when the substrate had been positioned by using the contact-positioning technique, to correct the rotational error when the substrate is placed onto the substrate stage through the substrate lift means, and to correct the offsets through the substrate stage after the substrate has been placed on the substrate stage. This enables simplification of the construction of the substrate stage and a quick alignment of the substrate.